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Vapor barrier in crawl space


Gen's Avatar
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08-01-15, 06:11 AM   #1  
Vapor barrier in crawl space

Hello, I discovered a dirt crawlspace in my house and I want to put down the 6 mil vapor barrier over it. It has been suggested that I consider putting the barrier on the underside of the floor joists, rather than on top of the dirt. The idea being that mold may form between the dirt and vapor barrier otherwise. I'd like some feedback on this. Thanks very much.

 
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08-01-15, 09:32 AM   #2  
You'll want to stop the moisture long before it gets to the joist level. 10 mil on the dirt, brought up on the sides and around all pillars, and adhered with HVAC mastic works well. Not much problem will exist under the vapor barrier, and you certainly don't want in on everything in the crawlspace such as pipes, water heater, air handler, wiring, etc. Stop it at the ground.

 
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08-01-15, 10:26 AM   #3  
Everything Chandler said, plus in cold climates I like to see at least 1" (preferably more) of rigid foam insulation on the inside of those crawlspace walls. Many like to use a foil faced rigid insulation where the foil provides the ignition barrier covering the rigid insulation. Dow has a good product.

Once you add the vapor barrier and seal it to the rigid on the walls, plus detailed insulation and air sealing for the rim joist all the way around, that space can be heated and will provide warmer floors through the house. I'll add a related link.
New Light in Crawlspaces | Building Science Corporation

 
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08-01-15, 12:50 PM   #4  
One more vote for the vapor barrier on the ground and tacked to the walls.

I'm with Bud on sealing the crawlspace to the outside and opening it to the house air as well.

 
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08-01-15, 01:15 PM   #5  
sealing the crawlspace to the outside and opening it to the house air as well.
In Michigan, definitely. Here in the South, we ventilate, so there are two schools of thought, and Building Science only takes one viewpoint.

 
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08-01-15, 07:28 PM   #6  
Thanks for the replies....You mentioned HVAC mastic. Is that for attachment to walls or for the seams. The exterior foundation walls are concrete block, the inner walls old irregular stone.

 
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08-02-15, 04:21 AM   #7  
Help me understand what you have. Was there an old stone foundation and they built a new block foundation outside of it or are those inner walls totally on the inside?

If the foundation wall surface you will be attaching to (the interior side of the outside walls) is a block wall that will be easier. If they left some old stone walls on the inside it will be difficult to get a good air seal. Let us know we will figure out something. Maybe pictures will help.

Bud

 
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08-02-15, 07:41 AM   #8  
Yes to your first question....."there was an old stone foundation and they built a new block foundation outside of it." The particular crawl space, is above one room, small--about 8 x 14. It was an addition. Inner original 2 walls are old stone circa 20's.

 
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08-03-15, 08:23 AM   #9  
Questions.....

1. Would it be better to just lay concrete over the dirt?
2.There are some minor gaps in the concrete block...should I fill them, and with what?
3. Given my old stone/big rock built foundation, should I be concerned with moisture or vapor diffusing through it?
3a. Is #3 an issue with concrete block?
4. At some point, I may just want to hire somebody to precisely deal with what I have. Given all the details and conflicting approaches, how might I best find someone who is rock solid to take care of this?

Thanks very much


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08-03-15, 10:32 AM   #10  
"who is rock solid to take care of this" LOL, it's those rocks that are getting in the way of the typical advice.

Best I can come up with would be to install the vapor barrier sealed over the floor space and then up the walls as best you can, but with a stone foundation exposed on the inside going up doesn't seem to add a lot of moisture protection. At least covering the floor would reduce that source.

As for any and all leaks through the outside block foundation they need to be sealed. Ideally, as illustrated in the link provided above, a layer of rigid foam on the inside would complete the encapsulation.

Now, I have seen a couple of stone foundations that were covered with spray foam. Technically you should then cover the foam with a special fire retardant paint ($$$) but may not be required everywhere. A closed cell foam would provide insulation and a moisture barrier. A company in your area that applies spray foam might be a good place to start asking questions, just leave your check book home for the first couple of visits as you gather information.

Bud

 
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08-06-15, 07:21 PM   #11  
OK thanks....and bare with me, my wife swears the house air is contributing to her asthma.......

I have the old stone/rock foundation walls around all of the exterior foundation (except for the block area I described above) Question: Is this also a major source of the musty house smell?--dirty musty air coming through the rocks?

 
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08-06-15, 07:47 PM   #12  
Recognize I'm some distance away so basically have to guess. IMO, a stone foundation wall would be a source of air infiltration, however, that doesn't mean it would have that musty basement smell. It might just be outside air finding a path through the stones and joints. Dirt floors however, are a common source of that musty smell.

Number one step would be the vapor barrier on the floor. As suggested before, talk to some of the spray foam companies about spraying those stone walls. At the same time they could also spray the rim joist. Always beneficial to talk with local contractors.

Now, for the asthma, consider an air filtration system. I assume pollen and outside contaminants are also part of the problem and even a modern tight home has enough leaks for all of the inside air to be replaced every 4 hours. Guessing again, but yours is probably exchanging all of its air every 2 to 3 hours. What some people will do is provide an incoming air source that is directed through a filtration system to deliver clean fresh air.

Your call as you know what the outside air is like.

Bud

 
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08-06-15, 07:54 PM   #13  
Thanks, this is all so critical for us. The dirt floor is really dry, but yes musty. Is there a benefit to do concrete floor instead of a vapor barrier? The actual dim is a meager 5 x 11 ft.

 
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08-06-15, 08:07 PM   #14  
If you were to pour a concrete floor you would also install a layer of plastic first. But no question, with that height, a concrete floor would be nice. It would also me permanent where the plastic ends up getting disturbed, even though you are not supposed to walk on it, and needs to be repaired or replaced at some point/s.

If the floor is nice and flat, or can be made so easily, I would go for the concrete, adds value and peace of mind. A floor down there doesn't have to be terribly thick. I would go for 3", but we have some experienced concrete people here so maybe they will jump in, Please.

Bud

 
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08-09-15, 08:03 AM   #15  
Thanks very much. I went into the crawlspace to seal gaps in the exterior concrete block mortar joints. This picture was what I found on one of exterior block wall.

What is a simple way, if needed, to repair this?Name:  0719151807-1.jpg
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Here's a better wider pic of this and the stone interior wall.Name:  0719151937-1.jpg
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It's about 5 feet wide.


Last edited by Gen; 08-09-15 at 08:54 AM.
 
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08-09-15, 08:46 AM   #16  
Sorry, I read that as 5' 11" tall, my bad. Since that isn't really usable space the plastic vapor barrier should be fine. Looking at the stone work they have filled most of the gaps so some detailed efforts to seal the VB to those walls should be fine.

It looks like they put the house down onto half blocks. To seal those I would spend some time with a caulking gun and hit all seams plus top and bottom.

Bud

 
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08-09-15, 08:58 AM   #17  
Yes it is 5 x 11 my mistake, I fixed the post info......the half block wall is only on the 5 foot side.

Do I have to fill the half block openings? Can I use spray foam for this?

Yes there's only one heat supply duct down there......(which I'd prefer to get out actually) So right I have no reason to go down there really. Thanks

 
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08-09-15, 09:16 AM   #18  
If you are going to insulate the walls spray foam would make an easy job of irregular surfaces like that. And you could spray the entire rock walls. Check your local lumber yards and box stores to see what brands are available locally. A note of caution, it apparently isn't as easy as it would seem. But it can be done, DIY. Others here may have used the home kits, I have not. The only cost savings (IMO) comes from the small job issues. Often times large contractors charge more for little jobs. It wouldn't hurt to get a couple of estimates.

There is no structural way to fill those half blocks, as far as simple and low cost, so caulking or spraying is all that is needed.

Bud

 
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08-09-15, 07:57 PM   #19  
Thanks I have to fill those gaps now. But before I put the vapor barrier down.......

I noticed on the dirt floor there are some miscellaneous wood chips and splinters mixed in with the dirt. If it helps, I'd say on a scale of 1 to 10 roughly the quantity is a 3. Do I have to get rid of these wood particles, lest they get wet, rot or potentially get moldy under the vapor barrier?

Second, when they installed the furnace 25 yrs ago they attached a joist return air plenum to suck into the crawl space. This seems odd to have musty crawl space air sucking into the furnace air and spreading through the house.

Me thinks I should block this off. Any thoughts? Here's a pic. Name:  0719151804.jpg
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08-10-15, 06:35 AM   #20  
Strange indeed. Yes, that needs to be changed, but return air volume shouldn't be reduced, so a return register will need to be added upstairs somewhere. Panned returns where they use joist or wall cavities instead of a metal duct are not good for your efforts to improve air quality. But making those changes may be more of a project than you had planned. However, it needs to remain on your concern list and if other work is being done, like replacing a wall, and a metal duct can be installed then the timing is right.

As for the wood chips under the plastic they are mold food, but the intent is for zero air to be leaking from under the plastic into the crawlspace. As for eliminating all food sources it can't be done with dirt down there anyway. Even pouring a concrete floor over a layer of plastic is just encapsulating whatever id below.

As for air quality in regards to air in the crawlspace, a radon system will pull air from below the plastic and exhaust it to the outside reducing the chances of air from below the plastic reaching the living space. It also reduces radon, which you can test to see if it is a concern.

Bud

 
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08-10-15, 06:49 AM   #21  
And maybe the weird plenum was going to be used for another addition that never happened? Anyway thanks. Radon....I do have a system about 6 feet lower and 15 feet away at the regular normal cement basement. Do I consider this effective on potential radon at the crawlspace?

 
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08-10-15, 07:32 AM   #22  
No, the Radon systems need to depressurize the space below the plastic or the concrete floor where currently installed. But my guess is, it could be extended to that additional area IF you can get it sealed tight enough and that could be tricky as air can move through dirt.

Example, if you buried a 2x6 on edge around the entire perimeter of that space and sealed the new plastic to the top edge so the inside area could not easily draw air from those walls, then at least the interior of that 2x6 frame could be attached to the Radon system.

That's is just a quick thought for an option.

Total return duct area should be equal or greater than all of the supply registers. That will give you an idea as to whether what you are looking at is extra or was part of the needed air flow. But in any case, it has been part of the air flow and if simply closed with no additional return area added you will be changing the system. We would need more system details to judge if that would be a problem.

Bud

 
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08-10-15, 07:01 PM   #23  
Thanks....you mentioned "Panned returns where they use joist or wall cavities instead of a metal duct are not good for your efforts to improve air quality." (You're referring to what I call return plenum I believe....)

Why is this a bad air case? I don't mind, if so, replacing them with 8" round metal.

BTW...I have 3 siblings in ME.

 
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08-10-15, 09:11 PM   #24  
The wood will tend to collect dust and other pollutants and cannot be easily cleaned. Again, return volume is important so substituting an 8" round for a larger rectangular duct may not be good. The real answer involves reviewing all of the supply and return paths to see how well they are balanced and if they are providing enough air flow. Older homes usually have trouble meeting one or both of those design requirements. As a note, when supply and return are not equal (balanced) the result is conditioned air is being forced out somewhere which requires outside air to be drawn in elsewhere.

I'm sure they like it here. I spent some time in NJ and it really helped me appreciate Maine. Chicago is the closest I've been to MI, but I'm sure all of MI doesn't look like a big city. Heck, you probably have more coastline than we do.

Bud

 
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08-11-15, 06:03 AM   #25  
Ok thanks. I'm going to have my HVAC guys evaluate things because there's other returns in another crawl space that seems to do nothing.

(Incredibly I actually have 2 other, possibly 3 other crawl spaces in this house....but they are concrete.)

Interestingly I spent spent 40+ years also in NJ.....central area near Brunswicks and Princeton.

 
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11-19-15, 08:00 PM   #26  
Hey Bud following up...I quit and I hired an energy company to deal with everything. They did foam on all the cellar and crawl space walls. Also vapor barriers and Reflectix on the crawl space floors. Then they sealed all the duct work. Thanks for all your replies. Here's pics before/afterName:  1015151314a.jpg
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11-20-15, 01:53 AM   #27  
Thanks for the follow up. I'm sure you will feel a difference with that job.

Bud

 
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